Heretofore, the restoration of posterior teeth, exhibiting decay, erosion, or fracture, has involved techniques such as fitting stainless steel crowns or caps over the damaged teeth. While the stainless steel crowns are well suited for structural and functional restoration of these teeth, and suit the expense capability of the lower income group of dental patients, they are unaesthetic. Another low cost alternative which is more aesthetic is the polycarbonate resin crown. This crown is not favored because such a crown exhibits poor wear resistance, inadequate retention, and is susceptible to cracking in service. It is therefore a desirable approach to the restoration of teeth to combine the functional properties of a stainless steel crown with the aesthetics presented by the polymeric crowns. One such approach which has been suggested is to veneer the steel crown with a thin layer of polymeric resin. This option, which still would provide an economical cost to the lower income dental patient, would combine the reliable function of the stainless steel dental crown and offer the normal appearance of a porcelain or plastic crown. Although low cost veneering has been employed for a number of years for the restoration of natural teeth, to mask defects such as developmental anomalies and tetracycline and fluorosis stains, a durable stainless steel-polymer veneer approach has not been developed.
In veneering natural teeth, the damaged teeth are isolated, cleaned, etched with phosphoric acid, and dried. A composite restorative resin is applied to the etched surface and cured. Adhesive interlocking of the resin with the etched surface of the tooth enamel results in a bond strength which is sufficient to accomplish a restoration having acceptable clinical durability. Unfortunately, stainless steel is not amenable to a similar approach because retentive etch pits cannot be adequately developed in stainless steel by acid etching as is normally used in natural teeth veneering.
Alternative approaches to produce a durable stainless steel-polymer veneer crown have involved chemical bonding techniques. The widely-used industrial adhesive agents, such as epoxy resins, phenol-formaldehyde resins, and acrylic and other dental approved resins have been employed in the past to adhere polymers to substrates. Unfortunately, the bond formed between the substrate and the polymer provides a weak structural interface relative to the structural stresses presented to the crowns when in service. Not only are the veneered products subject to cracking, flaking, and fracture, the veneer composite may exhibit undesirable permeability to gases such as water vapor, and liquids, thereby allowing further weakening of the veneer structure.
Other approaches to combine functional strength and aesthetics include pigmenting the steel crowns. For example, commercial crowns were at one time available with white pigmented layers. These crowns, however, were not successful because of the rapid loss of the pigment coating after oral placement.
U.S. Pat. No. 4,364,731 describes a more recent attempt to produce an aesthetic-functional composite which involves the procedure of providing a polymeric veneer over a stainless steel crown by applying an intermediate layer of inorganic oxide onto the cleaned stainless steel crown and then applying a layer of silane coupling agent over the inorganic oxide layer and then finally adhering the polymeric veneer to said silane coupling agent layer. This system, however, requires the elaborate time-consuming and generally more costly procedures of two intermediate layers between the stainless steel crown and the polymeric veneer. In addition, the use of silane materials is undesirable because they are generally considered toxic.
As can be appreciated from the foregoing, the need still remains for a dental crown which combines the structural strength of steel and the aesthetics of a polymer resin coating, without the necessity of expensive procedures and toxic chemicals to meet the demand of constant oral exposure. It is therefore a feature of the present invention to provide a dental crown composite, a method of making said dental crown composite, and of applying said dental crown composite which provides a stainless steel polymeric veneered dental crown having strong adhesive bonds between the substrate and the polymer resin and in addition may be installed in a single office visit.